These circuits may for example be used in active transmit/receive modules, such as those integrated within radars. More precisely, these modules comprise microchips able to phase shift and attenuate or amplify the signal received or to be transmitted. The amplification function is carried out by the amplification circuit which ideally must be capable of providing the necessary power required while guaranteeing good linearity of the transmit/receive chain.
An ideal amplification circuit must notably:                provide the power level required at output,        disturb the other circuits coupled upstream as little as possible,        be sufficiently linear for the intended applications, and        allow control of the output gain,        
all this with the lowest possible power consumption.
The amplification circuit is notably defined by its input impedance, its gain and its transition frequency.
So that the operation of the circuit is optimal, it is preferable for it to function in its linear operating zone, so as to benefit from maximum and controlled values of the amplification gain.
This linear operating zone is made up of the frequencies lying between 0 and the transition frequency Ft. The latter is defined by:Ft=Fc*Gmax,                where:                    Ft is the transition frequency,            Fc the cutoff frequency, and            Gmax, the maximum value that can be taken by the gain, in terms of current, of the amplification circuit.                        
To be able to deliver maximum output, it is necessary to function at frequency values for which the gain takes its maximum value, that is to say in the linear operating zone of the circuit.
Hitherto, in amplification circuits, working at high gain values implies a low input impedance, notably related to the use of a transistor self-biased by a current source, (for example a current mirror) and mounted in common emitter mode.
In this configuration (for transistors of judiciously chosen dimensions), the input impedance is expressed in the form:
                              Ze          =                      β                          2              ⁢                                                          ⁢              gm                                      ,                            (        1        )                            where:                    Ze is the input impedance of the amplification circuit, and            β is the gain of the self-biased transistor.                        
The disadvantage of a low input impedance is that:                it is harmful to the coupling with another circuit disposed upstream, and        it decreases the value of the transition frequency, and consequently reduces the extent of the linear operating zone of the amplification circuit.        
An aim of the invention is notably to solve these problems.